1. Field of the Invention
The present invention relates to a resonant inverter and a resonant power source unit that easily lower an operating voltage of a switch.
2. Description of the Related Art
U.S. Pat. No. 7,889,519 discloses an isolated resonant converter. The isolated resonant converter includes an isolation transformer provided on the side of a resonant rectifier, to isolate the output of a resonant inverter to be supplied to a load. In addition, U.S. Pat. No. 7,924,580 discloses a non-isolated resonant converter. The non-isolated resonant converter includes two switches having a phase difference of 180 degrees that are alternately turned on and off when supplying power to a load. Employing the resonant converter techniques disclosed in the cited documents enables provision of a resonant converter capable of supplying a large power with a low switch voltage. Further, U.S. Pat. No. 7,889,519 and U.S. Pat. No. 7,924,580 both teach that the switch voltage can be lowered by making the drain-source impedance of the switch highest at a driving frequency and 3 times thereof, and lowest at approximately twice of the driving frequency. Still further, IEICE Technical Report EE Vol. 107 (330), 2007, pp. 51 to 56 discloses a method of optimizing the drain-source capacitance of a switch when supplying power to a load resistance in a Class DE converter.
The resonant inverters disclosed in the cited documents include two single-end inverters coupled to each other, and are naturally capable of handling a power twice as large as the power that can be handled by a single-end inverter. In general, in addition, it is difficult to drive a high-side switch, for example used in a bridge circuit, at a high frequency in a high frequency inverter from the viewpoint of prevention of through current between the high-side switch and a low-side switch, because a dead time has to be surely suppressed to a short period. In contrast, the resonant inverter disclosed above only drives the low-side switch, and is therefore advantageous for expanding the power range of the inverter driven at a high frequency.
In the case of constituting the resonant inverter by the method according to the IEICE Technical Report, however, the switch operating voltage may become too high when the load resistance is determined first and then the drain-source capacitance is determined, because when the load resistance is high the drain-source capacitance becomes smaller. This is because when the drain-source capacitance is made larger to lower the switch operating voltage and the resonators are set up on the basis of the drain-source capacitance, resultantly the load resistance that realizes an optimal operation is lowered and, consequently, deviated from the optimal operation point when the resistance is high. Accordingly, a solution has to be provided to how to lower the switch operating voltage without affecting the drain-source capacitance of the switch, when the drain-source capacitance of the switch is determined on the basis of the load resistance.
The present invention has been accomplished in view of the drawbacks incidental to the conventional technique, and provides a resonant inverter and a resonant power source unit capable of lowering the switch operating voltage irrespective of the load resistance, and reducing the size and manufacturing cost of the apparatus.